Effect of synthetic human pancreatic growth hormone-releasing factors on plasma growth hormone concentrations in lactating cows.

Two fragments of human pancreatic growth hormone-releasing factor, hpGRF-(1-24)-NH2 and hpGRF(1-29)-NH2, were administered to lactating dairy cows in separate experiments. Each experiment involved a 4 X 4 Latin square with treatments being intravenous administration of saline (controls) or three doses of releasing factor (500, 1000, and 2000 micrograms). Administration of the 24-amino acid fragment increased plasma growth hormone concentration from baseline 3 ng/ml to peak 14 ng/ml within 10 min. Growth hormone concentrations returned to baseline by 60 min post treatment. The 29-amino acid fragment resulted in greater responses in both peak concentration (30 ng/ml) and persistency (greater than 240 min). Relationships between dose of releasing factor and response in growth hormone release (as measured by area under the response curve) were not consistent in most cows. Responses of concentrations of growth hormone in blood plasma that result from intravenous injection of up to 2000 micrograms of growth hormone-releasing factor (1-29) would not be sufficient to increase milk yield of cows.